An optical design is presented that enables the introduction of a defocus diversity function for image based WFS. The optical design is implemented by inserting a fixed or stationary lens into a converging beam ahead of the imaging focal plane. This lens insertion, in combination with various lens design forms, enables the creation of diversity defocus data without translating the imaging camera along the optical axis. The fixed-lens WFS technique is simpler to implement than translating the imaging camera and, therefore, simplifies the data collection process for focus-diverse phase-retrieval. The variation in F/# (focal ratio) is shown to have little or no effect on WFS results. Experiments performed at the NASA GSFC also show that a negligible optical residual is introduced while implementing the fixed-lens technique.